Add support for negative z-index in AnimationGroup (#4389)

* Add support for negative z-index in AnimationGroup

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Refactor animation unpacking logic for clarity

* Fix unpacking logic to handle Mobject instances correctly

* Fix mypy check

* Fix tests

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* fix: simplify AnimationGroup unpacking for moving mobjects

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Benjamin Hackl <devel@benjamin-hackl.at>

manim/scene/scene.py

@@ -903,7 +903,24 @@ class Scene:
         # as soon as there's one that needs updating of
         # some kind per frame, return the list from that
         # point forward.
-        animation_mobjects = [anim.mobject for anim in animations]
+        # Imported inside the method to avoid cyclic import.
+        from ..animation.composition import AnimationGroup
+
+        def _collect_animation_mobjects(
+            nested_animations: Iterable[Animation],
+        ) -> list[Mobject | OpenGLMobject]:
+            animation_mobjects: list[Mobject | OpenGLMobject] = []
+            for anim in nested_animations:
+                if isinstance(anim, AnimationGroup):
+                    animation_mobjects.extend(
+                        _collect_animation_mobjects(anim.animations),
+                    )
+                else:
+                    animation_mobjects.extend(anim.mobject.get_family())
+            return animation_mobjects
+
+        animation_mobjects = _collect_animation_mobjects(animations)
+
         mobjects = self.get_mobject_family_members()
         for i, mob in enumerate(mobjects):
             update_possibilities = [

tests/test_graphical_units/test_geometry.py

@@ -174,6 +174,27 @@ def test_ZIndex(scene):
     scene.play(ApplyMethod(triangle.shift, 2 * UP))
 
 
+@frames_comparison(last_frame=False)
+def test_negative_z_index_AnimationGroup(scene):
+    # https://github.com/ManimCommunity/manim/issues/3334
+    s = Square().set_z_index(-1)
+    scene.play(AnimationGroup(GrowFromCenter(s)))
+
+
+@frames_comparison(last_frame=False)
+def test_negative_z_index_LaggedStart(scene):
+    # https://github.com/ManimCommunity/manim/issues/3914
+    line_1 = Line(LEFT, RIGHT, color=BLUE)
+    line_2 = Line(UP + LEFT, UP + RIGHT, color=RED).set_z_index(-1)
+    scene.play(LaggedStart(FadeIn(line_1), FadeIn(line_2), lag_ratio=0.5))
+
+
+@frames_comparison(last_frame=False)
+def test_nested_animation_groups_with_negative_z_index(scene):
+    line = Line(LEFT, RIGHT, color=BLUE).set_z_index(-1)
+    scene.play(AnimationGroup(AnimationGroup(AnimationGroup(FadeIn(line)))))
+
+
 @frames_comparison
 def test_Angle(scene):
     l1 = Line(ORIGIN, RIGHT)